I have had a similar concept in mind, but I just cannot seem to figure out how they're doing this for the life of me. For one, crawling and scrapping is against their API TOS. Secondly, they are even pulling individual dish pictures (supposedly from instagram).
You can basically look up a dish, for example "Chicken Tikka". You will then see nearby places that serve chicken tikka and the rating for that restaurant (which Yelp allows), but to make things more complicated it shows what users over the web say about that dish and the specific restaurant.
A lot of people have asked a similar question everywhere but no one has the answers.
APIs. Many sites offer APIs to allow people to legally use their data. Yelp's is found at Yelp API overview Check out the Wikipedia article for more info.